Organic Materials showing electrically bistable behavior are very attractive for the developing of low cost, high density non volatile memory devices.
Organic Electrically Bistable Materials (OEBMs) can be defined as materials able to have a stable and reversible form upon either charge injection or charge extraction processes.
More exactly, in these materials when upon electrical stimuli the so-called threshold voltage Vth is reached, the resistivity switches from a low (high) to a high (low) value.
Thus, this phenomenon allows the material to exhibit two states of different conductivities at the same applied voltage.
FIG. 1 clearly shows one of the typical switching characteristics of organic electrically bistable materials based upon voltage variations.
The above property makes these materials appealing candidates for use in non-volatile memory applications. Indeed, in comparison to inorganic materials, organic materials allow the tuning of their properties by appropriate syntheses, to be low cost materials and also easy to process materials.
By examining the prior art on OEBMs, these materials can be grouped in the following main classes: Charge Transfer complexes, Organic Composite systems developed by UCLA University, Simple Organic Molecules, Polymers and Disodium salts of fluoresceine-based dyes, DDQ (2,3-dichloro-5-6-dicyano-1,4-benzoquinone) and TAPA ((+)-2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)propionic acid).
Charge Transfer (CT) Complexes are composed by a D-A system featured by:                D=metals with low ionization potential such as Cu, Ag, etc. or organic molecules such as e.g. tetrathiafulvalene (TTF)        
A=organic molecules such as e.g. tetracyanoquinodimethane (TCNQ), toluoylene dicarbamidonitrite (TDCN) etc. . . .
The use of CT complexes for producing switching device is known for example from the following six patents: U.S. Pat. No. 4,371,883; U.S. Pat. No. 4,507,662; U.S. Pat. No. 4,574,366; U.S. Pat. No. 4,652,894; U.S. Pat. No. 4,731,756; and Japanese Patent Publication No. 2001-345431.
Moreover, Cu:TCNQ CT complex has been extensively studied during the last twenty years by Potember et al. [R. S. Potember et al., Chem. Scr. 17, 219, 1981; Appl. Phys. Lett. 34, 405, 1982; 7 R. S. Potember, T. O. Poehler, A. Rappa, D. O. Cowan, and A. N. Bloch, Synth. Met. 4, 371, 1982. R. S. Potember et al., APL Techn. Dig. 1986, 7, 129; Appl. Phys. Lett. 1995, 67, 2241 and refs therein] and interesting results as materials for high-density memories devices have been found. Basically, these molecules show good stability and fast switching behavior at room temperature with ease processability (evaporation).
Nevertheless, these materials present some unsolved technological issues that block their employment in memory devices. Firstly, the electrical behavior of these materials have generally been found to be unstable, not reproducible and are strictly dependent on both structures and uniformity of films.
Several methods have been proposed to obtain a precise control of CT film structures [T. Oyamada et al., Appl. Phys. Lett., 2003, 83, 1253 and refs therein]. Secondly, the migration of Cu in the electrode materials has been also found with consequent contamination device problems.
Similar behavior to Cu:TCNQ has been also demonstrated for the compounds Ag:TCNQ [R. S. Potember et al., APL Techn. Dig. 1986, 7, 129; Appl. Phys. Lett. 1995, 67, 2241 and refs therein], Li:TCNQ [A. J. Gong and Y. Osada, Appl. Phys. Lett., 1992, 61, 2787], Ru:TCNQ [N. Watanabe et al., Phys. Rev. B, 1991, 44, 111]
Going on to CT complexes review, Xu et al. reports on electrically bistable behavior of all-organic D-A complexes composed by Melanine cyanurate (MC)+TCNQ and bis[2-butene-2,3-dithiolato(2-)-s.s′]-Ni (BBDN) TCNQ, respectively [W. Xu et al., Appl. Phys. Lett., 1995, 67, 2241]. The use of MC and BBDN instead of Cu avoid the surface Cu migration. The electrical tests showed transition time from high to low resistance <100 ns, but unfortunately, no switching reversibility suggesting a WORM (Write Once Read-Only memory) applications.
TTF-CA (a charge transfer complex of tetrathiafulvalene and tetrachloro-p-benzoquinone) have been also studied for use as electrically bistable compound for memories applications (see, for example, Japanese Patent Laid-Open No. 345431/2001 on pages 2 and 3, FIG. 1). Finally, to complete the review on CT complexes, very recently Infineon Technologies claimed the discovering a new CT complex exhibiting excellent properties to be employed in non volatile memory devices [R. Sezi et al., 2003 IEEE International Electron Devices Meeting, Paper #10.2].
The molecular structure of the complex has not been revealed. Electrical experiments performed on the devices based on these materials have shown resistance ratio between the high and low conductivity states higher than 100.
The memory cells exhibited non-volatile data retention of more than 8 months. Moreover, the material, that shows a temperature stability of higher than 250° C., survived several thousand write/erase cycles under ambient conditions without degradation.
With regard to Organic Composite systems developed by UCLA University, the UCLA compounds are known from the following patent applications WO/0237500 and US 2004/027849.
Moreover it is known that, UCLA researchers invented a novel organic electrical bistable device (OBD) comprising of a thin metal layer embedded within the organic material, as the active medium [L. Ma et al., Appl. Phys. Lett., 2002, 80, 2997; L. Ma et al., Appl. Phys. Lett., 2003, 82, 1419]. The devices are non-volatile, featured by low transition time and precisely controlled by the application of a positive voltage pulse (to write) or a negative voltage pulse (to erase).
Such an approach includes the presence of complicated structures that involve the necessity to increase the production process steps, and the scalability of such an approach may present problems. Indeed, the scalability perspective for the composite developed by UCLA researchers is much less exciting respect to that related to organic material with single component that can be, in principle, scaled down till single molecule.
With regard to Simple Organic Molecules, their use is known from the following patents: CN No 1,239,329; CN No 1,344,719, CN No 1,333,571, CN No 1,352,470 and CN No 1,363,936.
Moreover there are few examples reported in the literature of this class of materials [.G. Li et al., Appl. Phys. Lett., 2000, 76, 2532; Z. Y. Hua et al., Appl. Surface SCIENCE, 2001, 169-170, 447].
The first paper [.J. G. Li et al., Appl. Phys. Lett., 2000, 76, 2532] concerns a device based on 1,1-dicyano-2,2-(4-dimethylaminophenyl)ethylene (DDME). High-quality DDME thin films were grown by a modified vacuum deposition [Z. Q. Xue et al., Thin Solid Film, 1996, 288, 296] and tested in a sandwiched device Au/DDME/Au (cell area 2.25 mm2), fabricated on a Si substrate by usual vacuum deposition. The conductive state was stable at ambient atmosphere. Unfortunately, the switched regions resume the high resistance state when heated to 60° C. in vacuum for about 1 h, accompanied with the color return to brick-red. No reliability data are reported.
In the second paper [Z. Y. Hua et al., Appl. Surface SCIENCE, 2001, 169-170, 447] the molecular structures of 4-(2-pyridilazo)resorcinol, 1-(2-pyridilazo)-2-naphthol, glyoxal-bis-(2hydroxyanil) have been investigated. The resistivity of all these films can be transformed to 6-7 orders of magnitude (from ca. 1010 to 103-104 Ωcm) and, once if the films are in low resistivity state they cannot return to high resistivity state the applied voltage is switched off. The threshold voltage across these organic films with a thickness of 60 nm is 4±6 V and the transition time is 5±10 ns. No reliability data are reported.
With regard the use of Polymers, D. Ma et al. report results on the electrical characteristics of switching devices constructed using a poly(methacrylate) derivative with pendant anthracene chromophores, poly(methylmethacrylate-co-9-anthracenyl-methylmethacrylate) (10:1) (MDCPAC) [D. Ma et al., Advanced Materials, 2000, 12, 1063]. It has been observed that the Au/MDCPAC/Al device has a switching time from the OFF state to the ON state that is shorter than 0.5 ms and can switch several thousand times. No reliability data are reported.
Amongst the above-mentioned classes of compounds, disodium salts of fluoresceine-based dyes, as for example (Bengal Rose, Eosyn Y, Fluoresceine disodiumsalt), DDQ (2,3-dichloro-5-6-dicyano-1,4-benzoquinone) and TAPA ((+)-2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)propionic acid), are a very recent discover [A. Bandhopadhyay and A. J. Paj, Apl. Phys. Lett., 2003, 82, 1215-1217; A. Bandhopadhyay and A. J. Paj, J. Phys. Chem. B, 2003, 107, 2531-2536;].
Devices based on these molecules show electrical bistability with good retention time and cycles.
Correlation between switching devices based on different fluoresceine derivatives and interesting switching data storage properties is given in the Table here below.
MoleculeON/OFF RatioRetentionCyclesDisodium Bengale 105Tested for h>106Rose saltDisodium Eosin Y salt9800Tested for h>106Disodium Fluorescein   4Tested for h>106saltDDQ 104Tested for h>106TAPA 10410 hNotrewritable
For this reasons these materials represent an interesting technological compounds for electronics applications.
Moreover, these disodium salts of fluoresceine-based dyes (Bengale Rose, Eosyn Y, Fluorescein), are low cost, easy to process and operating at low drive voltage, and therefore particularly suitable to be employed in memory devices.
Nevertheless, disodium salts of fluoresceine-based dyes have however some still unsolved drawbacks.
The main drawback lies in that when electrical stimuli are applied, disodium salts of fluoresceine-based dyes present ionic currents. From electrical point of view, this is a very negative effect given that causes various drawbacks such as transient bistable behaviors, decreasing of ON/OFF ratios, and films damage.
Recently, Anirban Bandyopadhyay et al. proposed to produce supramolecular structures of a fluoresceine derivate, namely the Rose Bengal, via layer by layer electrostatic self-assembly (ESA), see A. Bandhopadhyay and A. J. Paj Advanced Materials 2003, 15. No 22, Nov. 17, 1949-1952. ESA films of Rose Bengal in a conducting polymer matrix have been obtained, which are used as part of integrated circuit. In particular, three water-soluble polymers as cation (poly(allylamine hydrochloride), poly(diallyldimethylammonium chloride) and poly(p-xylene tetrahydrothiophenium chloride)) and Rose Bengal as anion have been used.
However, this solution is not free of inconveniences. For example, the use of polymers as cationic layer provides low ON/OFF ratios and unsatisfactory performances of the integrated circuit.
In view of the above-outlined drawback of the state of the art, one technical problem underlying embodiments of the present invention is that of providing electrically bistable fluoresceine derivatives, avoiding the presence of ionic currents that are responsible of electrical drawbacks such as transient bistable behaviors and decreasing of ON/OFF ratios.